Exploring jet interactions in the quark-gluon plasma using jet substructure measurements in Pb-Pb collisions with ALICE

Abstract

Jets are generated in hard interactions in high-energy nuclear collisions. Jets propagate through the quark-gluon plasma (QGP) as the jet shower evolves; their interaction with the QGP, known as jet quenching, generates observable phenomena that provide incisive probes of the structure and dynamics of the QGP. For instance, medium-induced modification of jet substructure probes color coherence, and may be sensitive to differences in quark and gluon energy loss due to their different Casimir factors. Jet grooming can be used to focus on specific regions of phase space, isolating medium-induced effects on hard splittings in the jet shower. ALICE is well suited for such substructure measurements due to its precise charged-particle tracking, which enables high-efficiency measurements of narrow splittings in jets down to low transverse momentum. In these proceedings several recent jet substructure measurements in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV are reported, for both ungroomed jets and jets that have been groomed using the Soft Drop and Dynamical Grooming algorithms. Measurements of the groomed jet radius, $\theta_g \equiv R_g/R$; the groomed jet momentum fraction, $z_g$; and the groomed relative transverse momentum, $k_{\mathrm{T,g}}$ are reported. These measurements show direct evidence of modification of the angular structure of jets in the QGP, and provide new constraints on the search for large-angle scattering of jets off of quasi-particles by interaction with the QGP. New measurements of sub-jet fragmentation, generalized jet angularities, and jet-axis differences, which provide insight into the angular and momentum structure of modified jets are also presented. Comparisons to model calculations are discussed.